1. Field of the Invention
The present invention generally relates to a light scanning apparatus, a light scanning method, and an image forming apparatus.
The present invention relates more particularly to a light scanning apparatus (multi-beam scanning apparatus) that simultaneously scans a face (scanned face) with a plurality of light beams emitted by a light source(s), and to an image forming apparatus such as a laser printer, digital copier, a laser facsimile, and a laser plotter using the light scanning apparatus to write images.
2. Description of the Related Art
Light scanning apparatuses are used for laser printers, digital copiers, facsimile machines, and laser plotters, for example. The light scanning apparatus conventionally uses one light beam (single beam scanning method), but the light scanning apparatus that uses a plurality of light beams to simultaneously scan a single scanned face (multi beam scanning method) is being studied intensively.
Tandem type image forming apparatuses with a plurality of photosensitive media that can form color images are also being studied as well as image forming apparatuses with a single photosensitive medium. The tandem type image forming apparatus is provided with a plurality of drum-shaped or belt-shaped photoconductive photosensitive bodies along the path of a medium to which toner images are transferred. The toner images formed on the plurality of photosensitive bodies are transferred to the common medium (recording sheet) and synthesized into color images. The tandem type image forming apparatuses of the multi-beam scanning method are being studied too.
A scan line is the trace of the light spot scanning the scanned face. The ideal scan line is a line straight in the main scan directions, but the actual scan line is curved and/or tilts even if the light scanning apparatus is precisely assembled. Since the tilted scan line can be regarded as an aspect of the curved scan line, the curved scan line includes the tilt scan line in the following description.
In the case that a monochrome image is formed with the single beam scan method, even if the scan line is slightly curved, the curvature causes few problems since the slight curvature is not visible.
In the case that a scanned face is scanned with the multi-beam scan method, if all the scan lines curve in the same manner, the curvature causes few problems.
However, if the distances between a plurality of scan lines (scan line pitch) differ, the curvature does matter. If the scan line pitch within an image varies in the sub scan directions, the image is distorted or the density of the image is not uniform. The distortion of the image and the non-uniformity of the image density degrade the quality of the image substantially.
If the curvature of the scan lines is not uniform in a color image formed by a tandem type image forming apparatus, the curvature results in an unevenness in color, density, and hue.
In the case of the multi-beam scanning method, the scan pitch changes over time however precisely the light scanning apparatus is initially adjusted. In the case of the tandem type image forming apparatus, the change over time in the curvature of the scan lines is inevitable even if the curvature is equalized at the initial stage.
Especially, if a resin lens is provided in the light path from the light source to the scanned face, the resin lens may deform due to changes in temperature and/or humidity. Accordingly, the scan line pitch in a photosensitive body may change, and the curvature of the scan lines of different photosensitive bodies may differ.
The scan line pitch and the scan line curvature can be compensated by adjusting the light spot position on the scanned face in the sub-scan directions. There are various methods to adjust the light spot position.
Japanese Patent Laid-open Application No. 9-189873 discloses an invention in which the scan line pitch of the multi-beam scanning method is adjusted at high precision by reflecting light beams emitted by light sources with a galvanic mirror on the light path to a light deflecting unit and adjusting the angle of the galvanic mirror to move the light spot position on the scanned face in the sub-scan directions.
The above invention also proposes to compensate the scan line curvature and the scan line pitch by adjusting the light spot position in the sub-scan directions on the scanned face with a liquid crystal element.
The above methods effectively adjust the scan line pitch and compensate the scan line curvature, but inherit a side effect in that the adjustment of the light spot position also changes the light intensity of the light spot.
In the case of the invention described in Japanese Patent Laid-open Application No. 9-189873, if an aperture for shaping the light beam is provided between the galvanic mirror and the scanned face, the light beam adjusted in the sub-scan directions by the galvanic mirror may be partially blocked by the aperture, and consequently, the light intensity of the light spot may be reduced.
In the case that the liquid crystal element is used, a change in deflecting angle causes a change in the transmissivity of the liquid crystal element, and results in a change in the light intensity of the light spot.
In the case of the multi-beam scanning method, for example, if the light intensity of the light spots varies between the scan lines, especially the quality of half-tone images often degrades substantially. In the case of forming color images of the tandem type, if the exposure of the photosensitive bodies differs from one body to another due to the inequality of the light intensity of the light spots, the hue of the color images changes, and accordingly, the color reproducibility is degraded.
One of the methods to increase the writing speed at which the light scanning apparatus forms images is to increase the rotative speed of a polygon mirror that is a deflecting unit. This method has a limit in applicability due to the durability, noise, and vibration of a motor that rotates the polygon mirror and the limit in the modulation speed of the laser. To solve the above problems, a proposal is made for a light scanning apparatus that writes a plurality of lines simultaneously using a plurality of light beams (see Japanese Patent Laid-open Applications No. 2000-227563, 10-215351, and 9-189873, for example).
A multi-beam semiconductor laser is, for example, a semiconductor laser array in a package that emits a plurality of light beams. Such a multi-beam semiconductor laser can be used as a light source unit of the light scanning apparatus (multi-beam scanning apparatus). In the case of the semiconductor laser array, however, it is difficult to increase the number of channels due to the limitations of fabrication processes, to remove thermal and electrical crosstalk, and to shorten the wavelength of the light beam. The cost of the semiconductor laser array is still high.
On the other hand, single beam semiconductor lasers are used for general purposes in various industrial fields since short wavelength types are readily available and production cost thereof is low. Many proposals have been made for multi-beam scanning apparatuses that use the single beam semiconductor lasers or the above multi-beam semiconductor lasers as the light source and generate a plurality of light beams using a beam generating unit.
In the case that a plurality of light beams are generated using the beam generating unit, compared with the case in which the semiconductor laser array is used as the light source unit, the beam spot arrangement (scan line pitch) on the scanned face often changes due to environmental and timewise (elapsed time) change.
Accordingly, a method of compensating the beam spot arrangement on the scanned face by providing an electrically driven liquid crystal element in a light source unit or just after the light source unit, and deflecting the light beam in response to the electrical signal by a micro angle (from several minutes to several tens of minutes) is proposed (see Japanese Patent Laid-open Applications No. 2000-3110 and 2000-47214).
A description is made of the liquid crystal below.
The liquid crystal element used as the light path deflecting element includes a nematic liquid crystal layer of homogenous molecular arrangement sandwiched by two glass substrates. Inside of the glass substrates are formed transparent electrodes made of metal oxide. Generally, a uniform electrode is formed over the entire surface of a glass substrate (bottom face, for example) to form an electrical ground plane, and a patterned electrode that provides electric field distribution to the liquid crystal layer is formed on the other glass substrate (top face, for example). When an alternating voltage (rectangular pulses of several kilo Hertz, for example) is applied to the liquid crystal layer, nematic liquid crystal molecules having birefringence (a difference in refractive index along the long axis and the short axis of the molecules) are tilted along the electric field. The liquid crystal layer is equivalent to a medium having a locally different refractive index distribution in response to the electric field distribution for a single color light having a linear polarization parallel to the liquid crystal molecule (the direction of the long axis). Accordingly, the light transmitting threough the liquid crystal layer is spatially modulated in its wave surface or phase depending on the in-plane distribution of the applied voltage.
The electro-optic property of the liquid crystal element depends on the elasticity and the dielectric anisotropy of the liquid crystal used and the initial orientation angle of liquid crystal molecules without the application of voltage. The electro-optic property of a liquid crystal element with a small initial orientation angle (5 degrees or less) exhibits a steep rise (threshold) in a low voltage region. As the voltage increases, the electro-optic property becomes linear, then converges into a constant. The electro-optic property of a liquid crystal element with a large initial orientation angle has no threshold. The curve in the low voltage region can be approximated with a second order polynomial.
A proposal is made on the pattern of electrodes in which many long and narrow stripe-shaped electrodes are provided and a predetermined voltage is applied to each electrode. This structure is characterized in realizing a high speed response, a high spatial resolution, and degrees of freedom in the wave surface modulation (any complex wave surface modulation as well as the functions of beam deflection and lenses).
In the case that the light path deflecting element is constructed by the liquid crystal element, the linear region of the electro-optical property of liquid crystal and ladder shaped electrodes are used. The stripe-shaped long and narrow transparent electrodes of the current exposure technique having width and pitch depending on the resolution (about 1 μm) are formed in the beam exposing region. Both ends of the stripe-shaped electrodes are connected to gradient potential electrodes expanding horizontally on the outside. The electrodes are structured in such a manner that a plurality of ladder-type electrodes are arranged. The number (width) of bound long and narrow electrodes is determined by the maximum beam deflecting angle required in the region.
In the case that two different voltages selected from the linear region of the electro-optical property are applied to respective ends of the gradient potential electrode spreading in the crosswise directions, a blaze type phase profile is obtained and becomes equivalent to a micro prism array. The deflecting of the light beam perpendicularly incident on the liquid crystal layer is possible by controlling the applied voltage, and consequently the blaze angle.
In the case that the liquid crystal element is used as a light path deflecting element, as described above, it is necessary to provide an electric field distribution to the liquid crystal layer and to form a linear refractive index distribution (refractive index gradient). On the other hand, in the case that a large beam deflecting angle (maximum deflecting angle) is desired, the thickness of the liquid crystal layer needs to be increased. There is no spherical spacer material to be distributed in the liquid crystal layer for general use. The thickness of the liquid crystal layer for liquid crystal monitors is usually 5 μm. In the case that a liquid crystal layer with thickness of more than 10 μm, for example, needs to be secured, one needs to use low-grade spacer material with high diameter deviation. As a result, it is difficult to keep uniform thickness of the liquid crystal layer, and to sustain the linearity of the refractive index distribution of the liquid crystal layer, for example.
A tandem type full-color image forming apparatus is provided with four photosensitive body drums corresponding to cyan (C), magenta (M), yellow (Y), and black (K) disposed along the transportation surface of a intermediate transfer belt. A light scanning apparatus correspondingly provided for each photosensitive body drum scans the photosensitive body drum. An electrostatic latent image is formed on the surface of each photosensitive body drum, and is made visible with a toner of corresponding color. The toner images are sequentially transferred to a sheet of paper carried by the intermediate transferring belt, and a multi-color image is formed.
The scanning unit of the above light scanning apparatus is usually a polygon mirror rotated by a motor at a predetermined rotative speed. The light scanning apparatus includes a line cycle signal generating unit. The line cycle signal generating unit detects the laser beam from the scanning unit at a predetermined position, and generates a line sync signal. The laser beam is modulated by the image signal in synchronization with this line sync signal, and the image is written line by line. An intermediate transfer reference signal generating unit detects a mark on the intermediate transfer body at a predetermined position, and generates an intermediate transfer reference signal. An image forming operation of each color to form a toner image of the color on the photosensitive body drum is executed in synchronization with the intermediate transfer reference signal.
In such a color image forming apparatus, since the intermediate transfer reference signal and the line sync signal are not in synchronization, the phases of the intermediate transfer reference signal and the line sync signal greatly deviate as the number of the laser beams increases. Since the starting positions at which the images are written in the sub-scan directions deviate, the position of the toner image of each color deviates from the others, which results in the degrading of the multi-color images.
To solve this problem, a proposal is made on a color image forming apparatus characterized by a compensating unit that adjusts the starting position at which the image of each color is formed in the sub-scan directions and compensates for the color image deviation by switching the laser beams that first write the images on the photosensitive body drum depending on the phase relationship of the intermediate transfer reference signal and the line sync signal (see Japanese Patent Laid-open Application No. 10-239939).
However, the order in which the laser beams start writing changes randomly depending on the phase difference between the mark signal and the sync detection signal attached to the intermediate transfer belt. Accordingly, even in the case that the difference between the power of the plurality of laser beams is very small, the light energy exposing the photosensitive body drum of each color varies, even though the image of the color is the same. Accordingly, the color of the image becomes unbalanced (see Japanese Patent Laid-open Applications No. 2002-72606 and No. 2002-72607). According to an experiment, the color variation caused by a power deviation of 2% is already visible. The color variation is especially apparent in the case of reproducing grey color.
On the other hand, the image forming speed of such a color image forming apparatus needs to be improved. It is necessary to increase the rotative speed of the polygon mirror that is a scanning unit of the light scanning apparatus and/or to increase the frequency of the image signal in order to satisfy the need. However, if the rotative speed of the scanning unit and the frequency of the image signal are increased, the durability, noise, and vibration of the motor driving the polygon mirror and the modulation speed of the semiconductor laser cause problems. Accordingly, a multi-beam scanning apparatus is proposed that simultaneously scans a plurality of light beams and writes a plurality of lines.
The multi-beam semiconductor laser such as the semiconductor laser array that has a plurality of radiation points (radiation channels) in a package can be used for the multi-beam light source apparatus that emits a plurality of laser beams. However, it is difficult to increase the number of channels due to the restrictions of fabrication processes, to remove thermal and electrical crosstalk between channels, and to fabricate the semiconductor laser array that emits light beams of short wave length. Multi-beam semiconductor lasers having such desired features are expensive.
Light source apparatuses and multi-beam scanning apparatuses that are provided with single-beam semiconductor lasers as the light sources and generate a plurality of laser beams using a beam generating unit are also usable. The plurality of laser beams generated by the beam generating unit are often affected by environmental and changes over time. The arrangement of beam spots on the scanned face (beam pitch) consequently changes. To solve this problem, a method is proposed in which an electrically driven liquid crystal element is provided to compensate the beam pitch.
However, the liquid crystal element to adjust the light beam positions of the plurality of light beams on the scanned face causes deviation in intensity of the plurality of light beams. This deviation may result in the degrading in quality of images formed by the color image forming apparatus.